Background

The Red Imported Fire Ant (Solenopsis invicta) is one of the most concerning invasive species that has spread through the southern United States and many other places globally, especially at sites of habitat disturbance.  Fire ants have major economic impacts through costly attempts to manage them and from the loss of agricultural productivitiy. The ants also have major ecological impacts on ground nesting animals and by displacing native arthropod fauna.

S. invicta spread across the southern US since the 1930's to arrive in Austin around 1980. Prior efforts to control fire ants had focused on chemical treatments including aerial spraying of broad spectrum insecticides which were criticized by many, including Rachel Carson in Silent Spring. As the focus changed from ineffectual but harmful chemical treatments, scientists began to contemplate the use of biological controls.  

Early studies at BFL by grad student Don Feener showed the extent to which foraging by Pheidole ants was disrupted when they were attacked by tiny phorid flies. Subsequent studies found the same phenomenon happening in Tropical Fire Ants (S. geminata) and then Red Imported Fire Ants (S. invicta) when attacked by their respective phorid fly species. These small field observations led to large scale research programs at BFL and several other labs in the US. Subsequent progress on fire ant biocontrol has been made in the release of several phorid fly species and also through investigations into viruses and microsporidian pathogens. 

Goals

courtesy Alex Wild

The goal of our program is to provide a scientific basis for ongoing biocontrol efforts to establish a suite of phorid fly species and pathogens for fire ant biological control. The effectiveness of such biocontrol is expected to depend on the suite of introduced phorid species, coupled with other parasitoids and pathogens acting synergistically and having greatest impacts during periods of environmental stress.

A key feature of our efforts to achieve a lasting solution to the imported fire ant problem is to develop a fundamental understanding of the population and community ecology of native and imported fire ants, their parasitoid flies and pathogens, while simultaneously applying our findings to biological control efforts.

We seek to understand why ant species of the genus Solenopsis can vary so dramatically in native and introduced ranges in terms of their degree of ecological dominance and thus, their status as ecological and economic pests. Our research to date has centered on flies of the genus Pseudacteon (family Phoridae), specialized fire ant parasitoids, as host-specific biological agents from the fire ant home range in South America. Other parasitoids and pathogens are also being considered for biocontrol by our team and other research groups such as USDA.

 Research Program

1) Criteria for choosing the most effective Pseudacteon species for fire ant biocontrol. 

courtesy Alex Wild

During field studies in Brazil and Argentina, it was observed that some phorid species usually locate their hosts at mound disturbances while others are more likely to be found along fire ant foraging trails. Given the initial observations of the impacts of phorids on fire ant foraging, we surmise that the most relevant species to introduce will be those that attend foraging trails and potentially disrupt food acquisition by the colony. It is also known that each fly species has a preference for a particular size class of fire ant workers. Since the majority of foraging workers tend to be of the smaller size distribution, we further propose that the best candiate species will be flies that specialize on the more abundant, smaller wokers.  Additional considerations include similarities in climates between source areas and release areas. The fly species that match these criteria are P. obtusitus and P. nudicornis. 

2) Field releases of South American phorids and tracking their dispersal dynamics. 
Experimental releases of introduced phorids have been conducted at multiple sites, with the necessary permitting in place. Species successfully released and established include P. tricuspis, P. nocens, P. curvatus, P. obtusus. Other species that have been released without confirmed establishment include P. litoralis, P. cultellatus, P. obtusitus and P. nudicornis. All releases have all been carefully documented and the subsequent establishment and spread has enabled studies of how novel organisms spread into a landscape, a question of broad ecological interest. We have found that introduced populations of phorids go through a local growth period of about one year followed by a major expansion of up to 70km per year with the wind. However their spread against the wind is limited to less than 10km per year 

3) Interactions between Solenopsis ants and Pseudacteon fliess. 
Experimental field studies in Texas and Argentina have been conducted to explain how and to what extent Pseudacteon phorid flies reduce the competitive advantage of host fire ants. 

courtesy Alex Wild

4) Community ecology of Pseudacteon phorid fly species.
Comparative studies of over 20 phorid species from Argentina and Brazil have helped determine the manner in which these fly species differ in their detection and use of their hosts. At times, up to 10 phorid species have been found at one site, and this implies a considerable degree of niche specialization.

5) Laboratory analyses of Pseudacteon host specificity.
Our studies of attack behavior in the laboratory have determined that the degree of host specificity by phorid females and have shown that the candidate fly species are sufficiently host specific for release in USA. These studies, along with with similar efforts by USDA-ARS, have resulted in permits being issued for field releases of several Pseudacteon species.

6) Unexpected discoveries about phorid-ant interactions. 
Laboratory rearing of Pseudacteon phorids has been critical in the initial phases of deploying them for fire ant biocontrol, and then subsequently for controlled lab experiments into their fascinating biology. Unexpected discoveries by our lab and others have revealed environmental sex determination in some Pseudacteon species, and the occurence of a zombie-phase of parasitized ant workers just prior to fly pupariation. Other studies have examined the microbiomes and population genetics of the hosts and their parasitoids. In our research into the system, we have discovered and described over 13 new species of Pseudacteon.

7) Evaluating the impacts of introduced flies on the invasive fire ants.
While several studies show a decrease in resource acquisition by fire ants in the presence of phorid flies, the ultimate impact of phorid flies on fire ant populations has yet to be demonstrated in the field. We have initiated several studies including setting up baseline surveys of ant communities and fire ant densities to monitor long term changes that may take well over 10 years to show such effects. 

10) Evaluating the impacts and interactions between phorid flies and fire ant pathogens.
We are studying whether phorid flies may be involved with transmitting pathogens such as microsporidia and other pathogens between fire ant colonies. If this is confirmed, then the role of phorid flies becomes even more important as a biocontrol tool.

Our strategy has been to pursue all key parts of the problem simultaneously rather than to work on narrow pieces in linear fashion. This "parallel processing" approach depends heavily on involving a talented group of ecologists, molecular biologists and physiologists who can effectively attack their respective parts of the project, and coordinate with others in integration of findings to effect biocontrol of fire ants.